1. Field of the Invention
The present invention relates to audio spatial enhancement.
2. Description of the Prior Art
Audio systems for two-channel stereo have been demonstrated for over 100 years. Among the first published references is the 1881 demonstration at the Paris Electrical Exhibition of the transmission of sound via telephone from the Grand Opera of Paris to a listening room located several kilometers away. Listeners in the remote location were provided with two telephone ear pieces, each driven by a separate microphone located on the stage of the opera, through which the opera performance could be auditioned with remarkable clarity. Listeners were able to distinguish each individual singer and reported that the aural impressions changed with the relative positions of the singers, and their movements could be followed.
A major development in two-channel stereo sound was taught in British Pat. No. 394,325 (1931 by Blumlein). This patent describes a two-channel microphone system to control automatically the sound intensities of multiple loudspeakers such that the listener's ears detect low frequency phase differences and high frequency intensity differences which give the impression of a sound source emanating from the same direction as the original source. One embodiment involved a pair of nearly coincident bi-directional microphones which have their electrical outputs connected to generate a sum and difference signal pair. Additional circuitry to adjust the sum and difference signals has been used to alter the spatial qualities of the derived stereo audio signals. Boosting the difference signal to broaden the perceived stereo image was used more extensively starting in the late 1950's after stereo recording and broadcasting was introduced.
A side effect of the introduction of two-channel stereo reproduction systems in the consumer marketplace was a growing interest in mono-to-stereo conversion schemes that would create pseudo-stereo signals from a pre-existing monophonic recording. Several well-known methods include (a) sending the mono signal directly to one output channel while sending a slightly delayed or phase shifted version to the other channel, (b) sending a low pass filtered version of the mono signal to one channel and a high pass filtered version to the other channel, (c) sending a comb filtered version of the mono signal to one channel and a version processed by a complementary comb filter to the other channel, and (d) creating an incoherent pair of output signals by passing the mono input signal through separate channels of a stereo reverberation system.
Prior art stereophonic enhancement inventions combine left (L) and right (R) channels with processed versions of L+R and L-R in empirically determined proportions. All of these systems therefore suffer from one or more of the following drawbacks:
(a) The enhancement process is based largely upon empirical results or trial-and -error parameters, which makes systematic improvements and alterations unwieldy. PA0 (b) The existing schemes typically involve a summation using the L-R signal, which creates inverted components (-L in the right and -R in the left) which cannot be controlled separately from the L and R signals. PA0 (c) The stereo enhancement is achieved at the expense of noticeable timbral coloration or delay/interference effects that destroy the natural sound of the signal. PA0 (d) Use of L-R and L+R in the enhancement process requires elaborate feedback and control mechanisms because of the rapidly varying behavior of the sum and difference signals. PA0 (e) The inherent complexity of the sum and difference approach requires special hardware or substantial computational resources to implement.
A need remains in the art of spatial enhancement for apparatus and methods for increasing the perceived dimensions of the sound field while overcoming the above disadvantages.